A well-differentiated adenocarcinoma was suspected on the basis of the initial forceps biopsy specimen (Fig. 3A). Because the patient primarily preferred the endoscopic treatment, endoscopic submucosal dissection (ESD) was performed. Histopathological examination of the ESD specimen revealed high-grade dysplasia with clear resection margins (Fig. 3B). However, forceps biopsy specimens obtained from a suspected subepithelial lesion near the post-ESD ulcer were identified as synchronous well-differentiated adenocarcinoma (Fig. 4). Therefore, the patient underwent total proctocolectomy. Pathological examination of the colectomy specimen confirmed the presence of two foci of well-differentiated adenocarcinoma invading the deep submucosal layer (sm3 level) of the rectum without lymph node metastasis (Fig. 5).

The risk of colorectal cancer (CRC) is increased in patients with long-standing UC,¹ and surveillance colonoscopy for UC is imperative for the timely detection of precancerous dysplasia and CRC at a curable stage. Recent surveillance guidelines recommend targeted biopsy for suspected dysplastic lesions instead of random biopsy in patients with UC.² Panchromoendoscopy can enhance dysplasia detection in UC compared with white light endoscopy in the standard definition system, and may improve the dysplasia detection rate in the high-definition endoscopy system in patients with UC at a higher risk for CRC.^2,3 Moreover, although time-consuming, it can visualize subtle mucosal abnormalities that are not discernible under white light endoscopy. However, in practice, the detection of UC-associated dysplasia or CRC is challenging. Neoplastic pit or narrow band imaging (NBI) patterns help in discriminating dysplasia or cancer in UC; nonetheless, their diagnostic performance seems suboptimal compared with that of pit or NBI patterns for colorectal neoplasia in non-colitic patients.⁴ Diffuse, chronically inflamed colonic mucosa can confuse endoscopists in discerning the true dysplasia from inflammatory sequelae. Moreover, unlike sporadic colorectal adenoma or cancer, dysplastic changes in colitis-associated lesions occasionally arise from the base of the crypt.⁵ Consequently, neoplastic pit or NBI patterns may not be obvious on the surface of colitis-associated dysplasia or cancer. Dysplasia and CRC in this case presented as subepithelial lesions on colonoscopy, although the lesions themselves arose from the glandular epithelium. Therefore, during surveillance colonoscopy of patients with UC, targeted biopsies should be performed not only from lesions suspicious of dysplasia or cancer but also from lesions that are inexplicably different from the surrounding mucosa.³